The Genetics of Inheritance

Why do we look the way we do? All organisms are made of cells. Inside of these cells, a chemical called DNA contains the “instructions” that control the way we look. DNA

DNA: the instruction manual The segments in the DNA that carry the “instructions” are called GENES. Every individual has two of every gene. There is more than one version of each gene. The different versions are called alleles. Blue Eyes Brown Eyes Brown Hair Black Hair Fair Skin Olive Skin

Dominant & Recessive Human Traits Earlobes: Widow’s peak: Free Attached # Students w/ free lobes attached lobes # Students w/ a widow’s peak # Students w/out a Widow’s peak Count the number of students with free or attached ear lobes & with a widow’s peak or without a widow’s peak. In general, you will find that more students have “free” ear lobes vs. attached & more students will have a widow’s peak vs. without a widow’s peak because these are dominant traits. See if the students can figure this out without telling them. One allele is dominant and the other allele is recessive. The dominant allele covers the expression of the recessive allele.

Genotype & Phenotype The observable traits of an organism is called the phenotype. # Students w/ free lobes # Students w/ attached lobes The genotype is the combination of all of the different alleles that make up an individual Free or attached ear lobes are the phenotype. The genotype is the combination of alleles that a person has and are represented with Capital letter for the Dominant trait & small letters for the recessive trait. So, Students with the dominant phenotype (free lobes) could have the genotype LL or Ll (where L=free lobes & l = attached lobes) and the students with the attached lobe phenotype have the genotype ll.

Incomplete Dominant Traits Blood types: Flower color: In incomplete dominance, you can either observe both alleles (as with the ABO blood type– a person with an A allele & B allele has an AB phenotype because neither allele is dominant over the other) or you observe a blending of alleles (for example, in some plants, flower color). In incomplete dominance, the dominant allele does not cover up the expression of the recessive allele.

Why do we look like our parents? Aa aa Aa aa Aa aa Aa aa In reproduction, each parent donates one of their alleles for every gene to their offspring. The probability that either allele will be passed on to the offspring is ½. And if parents have more than one child, the probability will still be ½..that is these are independent events.

Punnett Squares A diagram that is used to predict the genotype and phenotype of the offspring based on the genotype of the parents. pp It’s a probability problem! How do geneticists use probability & proportions to predict what the genotype & phenotype of the next generation may be? They use a tool called a Punnett Square. p P Pp pp Pp ½ purple pea plants; ½ white pea plants

Punnett Squares What is the probability that this couple will have a child with brown eyes? 2. What is the probability that this couple will have 3 children with brown eyes? Bb Bb BB= bb= Bb=

More Punnett Squares There are 10 guinea pigs in a litter. Five of the guinea pigs have long hair, and five have short hair. Long hair is dominant and short hair is recessive. What are the genotypes of the parents? Genotype for long hair: Genotype for short hair: Explain that this problem is solved by working backwards….First assign a letter to represent long hair (dominant) could be “L” and short hair (recessive) could l. We know that the guinea pigs with short hair must have 2 short hair alleles since it is a recessive trait. There are 5 guinea pigs with short hair. What proportion of 10 is 5? Fill in the bottom 2 boxes in the punnet square with “ll” The other 5 guinea pigs are “L_” Fill in the top 2 boxes in the punnett square with “L_” and work backwards to figure out the genotype of the parents. Also mention that usally the proportions of offspring will not be perfect—for example, there may have been 6 & 4 guinea pigs instead of 5 & 5—so often, you must estimate or round to the nearest fraction.

Applications of math in genetics Selective breeding—The selection of certain seeds or animals for reproduction so that the next generation has the same desirable traits.